Cultures of two Dipteran flies (Calliphora vicina (R-D) and C. vomitoria (L.)) were established to answer questions in regards to responses to thermal and desiccation stress, effects of diapause and the mechanisms which underpin diapause. The findings are divided in to two sections. Unequivocal new findings – Calliphora vomitoria were seen to depend on water being present in culture medium for increased survival. Furthermore, C. vomitoria were found to have lower desiccation resistance than C. vicina. Larvae of C. vicina and C. vomitoria showed different cold tolerance strategies, with C. vicina being freeze-avoiding and C. vomitoria ‘partially’ freeze-tolerant. Metabolomics, using \(^1\)H-NMR, revealed that diapause and non-diapause had distinct metabolic profiles. Diapause larvae were seen to reduce energy synthesis from the Krebs cycle and increase glycolysis. Calliphora vicina and C. vomitoria also exhibited different diapause phenotypes; C. vicina entered a maternally regulated facultative diapause as an L3 larvae, Calliphora vomitoria had a less distinct diapause, with maternal conditions having little effect. Speculative new findings - Despite the above differences C. vicina and C. vomitoria were able to produce a viable cross, though field fresh C. vomitoria were not used, as such it cannot be confirmed if this could occur in the wild. Increased temperatures due to climate change may affect both phenology and survival of insects; C. vicina was seen to have a delayed induction to diapause and a reduction in the proportion entering diapause. Diapause conferred increased cold tolerance; therefore those insects that overwinter not in diapause may suffer increased mortality.